I do think Metallica's progress as musicians is more logical and gradual than "they made metal and then sold out."
I'm totally derailing the topic, but the way I see it is this:
A lot of their style changes can be pinned on both line-up changes and the way rock/metal evolved at the time. The first two albums relied heavily on Dave Mustaine's stylistic influence, and when that started to wane, Cliff Burton had a huge impact artistically. Even if his songwriting contribution on Master of Puppets wasn't as big as on Ride the Lightning, there was still this sense of 'if Cliff doesn't like it, it ain't happening'; at least that is what Kirk Hammett later said. After Burton's death, Metallica ceased to be a full band and it was largely Hetfield and Ulrich (mainly the latter) just telling the others what to do.
Hence why And Justice For All sounds like ass production-wise; they had no one left who could step in and just say 'no, that idea sucks', which is why Metallica albums also became longer and longer with seemingly no one there to direct/edit things into a coherent, strong unit. Fortunately, AFJA is also an album where Metallica really challenge themselves in terms of composition and playing, so it's still a very creative album. They're really trying to prove themselves as musicians, and you get the idea they're punching a little over their weight, which I find very exciting.
With the Black Album, they had little left to prove as musicians and dumbed everything down to a large degree, which I still find a very logical thing to do for an artist; do the opposite of what you did on your previous album, lest you become a one-trick pony. As an added bonus, Bob Rock proved to be the guy who not only delivered what I consider to be the best production in pretty much any album, but also wasn't afraid to confront Hetfield and Ulrich creatively. There is a scene in that documentary they made of the recording process where Ulrich tries to pull the same trick as on AFJA and tells the producer (Bob Rock in this case) to turn the other instruments down, and Bob Rock's just like "we tried that and it sounded like shit, so no."
After that album, Rock seemingly got too close to the band, and Metallica entered the pretty much inevitable midlife crisis phase of their careers, so nothing they put out since then held anywhere near the weight of their first 5 albums.
Master of Puppets is imo the first Metallica album that contains filler, or at least contains clear stand-out tracks that carry the entire album.
If I had to make a top 5 it'd look like...
1. Ride the Lightning
2. Kill 'Em All
3. Master of Puppets
4/5. And Justice For All/Black Album
AFJA and Black Album just really depend on my mood and if I'm more able to tune out the bad production/mixing of the former or the simplistic songwriting of the latter.